I. Field
The following description relates generally to communication systems, and more particularly to a method and apparatus for scheduling transmissions in Spatial Division Multiple Access (SDMA) network systems.
II. Background
In order to address the issue of increasing bandwidth requirements that are demanded for wireless communications systems, different schemes are being developed to allow multiple user terminals to communicate with a single access point by sharing the channel resources while achieving high data throughputs. Multiple Input or Multiple Output (MIMO) technology represents one such approach that has recently emerged as a popular technique for the next generation communication systems. MIMO technology has been adopted in several emerging wireless communications standards such as the Institute of Electrical Engineers (IEEE) 802.11 standard. IEEE 802.11 denotes a set of Wireless Local Area Network (WLAN) air interface standards developed by the IEEE 802.11 committee for short-range communications (e.g., tens of meters to a few hundred meters).
In communication systems, medium access control (MAC) protocols are designed to operate to exploit several degrees of freedom offered by the air link medium. The most commonly exploited degrees of freedom are time and frequency. For example, in the IEEE 802.11 MAC protocol, the time degree of freedom is exploited through the CSMA (Carrier Sense Multiple Access). The CSMA protocol attempts to ensure that no more than one transmission occurs in a neighborhood of potential high interference. The frequency degree of freedom can be exploited by using different channels.
Recent developments have led to space dimension being a viable option. Spatial Division Multiple Access (SDMA) can be used for improving utilization of the air link by scheduling multiple terminals for simultaneous transmission and reception. Data is sent to each of the terminals using spatial streams. For example, with SDMA, a transmitter forms orthogonal streams to individual receivers. Such orthogonal streams can be formed because the transmitter has several antennas and the transmit/receive channel consists of several paths. The receivers may also have one or more antennas, such as in MIMO or Single Input-Multiple Output (SIMO) antenna systems. In one example, the transmitter may be an access point (AP) and the receivers may be stations (STAs). The streams are formed at the AP such that a stream targeted at a particular STA, for example, may be seen as low power interference at other STAs, which will not cause significant interference and not be ignored.
With multiple STAs and APs capable of simultaneous transmission in the SDMA WLAN system, efficient scheduling for downlink transmissions, which are transmissions from the AP to one or more STAs, and uplink transmissions, which are transmissions from one or more of the STAs to the AP, is desirable. Additional complexities are involved when scheduling simultaneous transmissions to and from multiple STAs; where each STA may have different pathloss and/or Quality of Service (QoS) requirements. Further, as each STA and AP may have different numbers of transmit and receive antennas, complexities added by hardware have to be considered during scheduling.
Consequently, it would be desirable to address one or more of the issues described above.